Upon learning another of the guests, Professor Peach, had been killed in the library, the Doctor and Donna pass themselves off as investigators and find a fragment of a burnt birth certificate.
While searching a room that Lady Eddison shut herself in for six months in 1886, Donna is attacked by a giant wasp, which escapes and later kills the housekeeper, Miss Chandrakala.
Later that night, the Doctor attempts to expose the Vespiform, but a thunderstorm knocks out the power, leading to Eddison's necklace being stolen and her son Roger Curbishley being killed in the darkness.
The Doctor and Agatha gather the remaining guests and expose two of them, Robina Redmond and Reverend Golightly, as the Unicorn and the Vespiform respectively.
By adulthood, Golightly discovered his true nature and, using knowledge from Agatha's books that he absorbed due to the necklace, sought revenge on Eddison for denying him the truth.
Roberts based the episode on his favourite Christie works: Crooked House, which focuses on secrets within an aristocratic society, and the 1982 film adaptation of Evil Under the Sun.
We don't really see posh people on television anymore, except at Christmas", and "there's something funny about the veneer of upper class respectability and the truth of any family underneath".
[3] In an email conversation with journalist Benjamin Cook, Davies admitted he had initially added a reference to the original title of And Then There Were None, Ten Little Niggers, but decided it was too risky.
[4] The casting of Fenella Woolgar as Agatha Christie was made at the suggestion of David Tennant, who had previously worked with her on Bright Young Things and He Knew He Was Right.
A framing device featuring the aged Agatha Christie (played by Daphne Oxenford) trying to recall the events that took place during her disappearance was deleted because the producers felt it diminished the story's urgency.
He claimed that style and substance "merge wonderfully in 'The Unicorn and The Wasp'", praising Gareth Roberts for his "deceptively frivolous story" managing to "successfully juggle many elements including the real life disappearance of Agatha Christie, a giant alien bug and a recreation of the classic 'whodunnit' murder mystery plot."
However, he claimed that there was also an "air of predictability about all of the tongue-in-cheek references to Agatha Christie's body of work", citing prior serials that contain famous authors such as The Unquiet Dead and The Shakespeare Code.